1. Field of the Invention
The present invention relates to a color correction device which adjusts the color components of an image signal containing color information in a color image reproducing machines.
2. Description of the Related Art
In digital color image reproducing machines such as color copying machines, color facsimile devices or color video printers, an original color image is converted by a photoelectric conversion element to R (red) G (green) and B (blue) component density signals, which are then converted digitally to corresponding color image data components Dr, Dg and Db. These image data components Dr, Dg and Db are then converted to recorded density data components Dy, Dm and Dc of recorded colors Y (yellow), M (magenta) and C (cyan), and the respective recorded densities are determined on the basis of the Dy, Dm and Dc.
Theoretically, the original image and print images are the same in color. However, actually, if the R, G and B signals obtained from the original are solely converted to C, M and Y signals and printed in three color toners, a large difference will be produced between the original color and the printed color because (a) the spectral characteristic of color separation filters used in the image read unit are actually not ideal, (b) the color of each of the toners used for printing is also not ideal, and (c) the color obtained by the principle of subtractive color mixture for laying a plurality of print colors sequentially one on top of another is different from the actual printed color. In order to avoid such discrepancy, when the R, G and B signals are converted to the C, M and Y signals, a masking process is performed to correct the colors. However, an "additivity rule of densities" does not actually hold due to the influence of surface reflection, etc. As a result, correct color correction cannot be performed.
The masking techniques are known, for example, in (1) Japanese Laid-Open Publication No. 59-161981 (British Patent No. 2145598) and (2) Japanese Laid-Open Publication No. 62-47273. In the techniques of the reference (1), nine coefficients used in the masking process are separately adjustable in a plurality of stages. By changing those coefficients, the contents of the color correction by the masking process are changed and as a result the colors to be printed are adjustable. In this technique, however, since the respective masking coefficients do not correspond to the basic colors of Y, M and C in one-to-one relationship, it is not clear which coefficients should be adjusted in what manner in order to obtain a desired particular color. Therefore, adjustment is required to be repeated many times by trail and error. If the color tone of a particular color is preferentially changed, a balance among Y, M and C colors may be undesirably lost and achromatic color portions such as black and gray portions of the image may often be colored.
The techniques of the (2) reference are intended to correct the colors by referring to the contents of a memory table and changing a part or the whole of the memory table to adjust the colors. However, in the particular reference, it is necessary to prepare a memory having a large capacity corresponding to the degree of freedom of color adjustment. In that case, therefore, if the degree of freedom of color adjustment is heightened, the device would undesirably become expensive.
In these types of devices, the user must adjust colors as needed in addition to the basic color correction because of the aging of the color characteristic of the device itself, variations in the characteristic of the individual devices, the user's own taste in colors, and intentional requirement for color change, etc.
In order to facilitate the user's adjustment, the inventor proposed a color copying machine in which the user inputs data on a color to be changed and information on the resulting color to a color correction device which sets a color correction parameter on the basis of these inputs, as disclosed in U.S. Ser. No. 260,277 filed Oct. 20, 1988. In this color copy machine, a hue is divided into a plurality of areas, each having an allocated color correction parameter, which is adjusted so that the color of only a desired hue area is adjusted, and thus recorded color adjustment does not act on a hue area deviating greatly from the desired hue area and hence recorded color adjustment is easy and performed with high reliability.
The hue area judgement circuit disclosed by the aforementioned application has a complicated structure and it is difficult to simplify the structure of the hardware and processing logic to make the whole device more inexpensive.